The present invention relates generally to the solid state polymerization (SSP) processing of polyethylene terephthalate (PET) material, as well as the material produced by such process, and more particularly to the solid state polymerization (SSP) processing of post-consumer and non-post-consumer polyethylene terephthalate (PET) material, especially for use in connection with the fabrication or manufacture of high-performance strapping, as well as the high-performance strapping produced by such process.
Post-consumer polyethylene terephthalate (PET), which is primarily provided by or derived from plastic soft drink bottles, can be readily obtained from material recovery facilities. Such material exhibits relatively low and heterogeneous intrinsic viscosity (IV) values, and in the past, this characteristic has prevented PET from being directly used to produce products, such as, for example, high-performance plastic strapping, which in fact require relatively high and homogeneous intrinsic viscosity (IV) values. It was one of the discoveries of the invention disclosed within the aforenoted related U.S. patent application Ser. No. 08/794,538 that such heterogeneity of the intrinsic viscosity (IV) values of the PET material did not in fact adversely affect the production of high-performance strapping, and the present invention comprises a further improvement upon the processing of such PET material.
In accordance with prior art processing techniques, the PET material, whether post-consumer and/or non-post consumer material, was initially chopped into flakes and chunks, and the flakes and chunks were extruded into pellets. The chopped PET material had a relatively low and wide range of IV values because the various soft drink bottles, for example, were manufactured by different companies using different materials exhibiting different IV values. The IV values were typically within the range of 0.65-0.80 dl/g. In accordance with such prior art processing techniques, it was further believed that in order to make a high-performance product, such as, for example, high-performance plastic strapping, from such post-consumer PET materials, it was necessary that the materials exhibit or achieve a relatively high and narrow range of IV values after the solid state processing which therefore required, as an initial step, the pelletizing of the flakes before commencement of the solid state polymerization. When the PET pellets are then subjected to solid state polymerization (SSP), the pellets would have their IV values raised and exhibit a relatively high and narrow range of IV values whereby such enhanced pellets could then be used to produce high-performance products, such as, for example, high-performance strapping.
As noted hereinabove, in accordance with the noted prior art processing techniques, the prior art solid state polymerization (SSP) of the PET materials commenced with pellets of uniform geometry. Such prior art solid state polymerization (SSP) of the pellets, however, required an inordinate amount of time, that is, approximately twelve to nineteen hours, to complete in order to produce the desired strapping, and it was not appreciated, until the invention disclosed within the aforenoted U.S. patent application Ser. No. 08/794,538, that a heterogeneous mixture of flakes and chunk-like PET materials could undergo direct solid state polymerization, without necessarily being initially pelletized, to the same or higher average IV values as those of the prior art pellets, and in a significantly faster manner, that is, upon the order of one-quarter the time required for the solid state polymerization of the pellet materials.
More specifically, while the resulting prior art strapping exhibited average IV values which were not greater than 0.90 dl/g, high-performance plastic strapping fabricated in accordance with the processing techniques disclosed within the aforenoted U.S. patent application Ser. No. 08/794,538 exhibited average IV values which were greater than 0.90 dl/g. Therefore, in accordance with the teachings of the invention embodied within the aforenoted U.S. patent application Ser. No. 08/794,538, high-performance plastic strapping could be commercially manufactured in an economical manner using PET materials, having a relatively wide distribution of IV values, and as a result of undergoing solid state polymerization directly from flaked materials which do not have to be initially pelletized.
While the solid state polymerization processing of PET flake materials, and the production of the resulting high-performance plastic strapping, as disclosed within the aforenoted U.S. patent application, having Ser. No. 08/794,538, has been quite successful and has resulted in the production of highly suitable plastic strapping, it has been discovered that the process can be further improved from an efficiency and material flow-through production basis with decreased production downtime. For example, when the PET materials, which are to be used in accordance with the processing techniques of the aforenoted invention disclosed within U.S. patent application Ser. No. 08/794,538, are in fact derived from plastic soft drink bottles, it has been discovered that as a result of the blow-molding manufacturing techniques attendant the fabrication or manufacture of soft drink bottles, the recycled PET bottles comprise essentially two different types of materials, that is, substantially crystalline wall sections and substantially non-crystalline neck sections, and accordingly, such different materials must be handled or processed differently. More particularly, the wall sections are preferably to be retained and utilized within the solid state polymerization (SSP) process, while the neck sections are preferably to be discarded from the solid state polymerization (SSP) process in accordance with the following.
It is known, for example, that temperature control of the solid state polymerization (SSP) process is critical if the temperature level of the solid state polymerization process is too low, the polymerization reaction will be too slow, while if the temperature level of the polymerization process is too high, the chips will melt or soften and thereby form clumps which will clog or jam the feeders or other components of the processing equipment. Since the wall portions of the recycled soft drink bottles are essentially crystalline, they react quickly within the solid state polymerization unit so as to desirably increase the molecular weight or intrinsic viscosity (IV) of the PET batch or charge, however, since the neck portions of the recycled soft drink bottles are substantially non-crystalline, they react quite slowly, if at all, within the solid state polymerization process in connection with the build-up or enhancement of the molecular weight or intrinsic viscosity properties of the PET materials being processed. Consequently, it is desirable from a processing efficiency point of view to have the batch or charge of PET materials within the solid state polymerization vessel to comprise more of the crystalline wall sections of the recycled PET materials than the non-crystalline neck sections of the recycled PET materials. In addition, and even more importantly, such non-crystalline neck portions or segments of the PET materials will crystallize rapidly within the polymerization processing vessel thereby emitting a significant amount of heat of crystallization. This heat of crystallization can be large enough to undesirably raise the temperature level of the process within the solid state polymerization vessel such that the PET materials tend to become sticky and agglomerate thereby forming clumps or chunks which will impede the flow of the materials within the polymerization vessel as well as clog or jam the various vessel components.
A need therefore exists in the art for effectively dealing with non-crystalline portions of recycled PET materials whereby, for example, such non-crystalline portions or segments of the recycled PET materials can be effectively removed from the batch or charge of PET materials to be fed into the solid state polymerization vessel such that processing or flow-through problems of the materials attendant temperature excursions, which would otherwise develop as a result of the rapid crystallization of such non-crystalline materials within the polymerization vessel and the consequent generation of a significant amount of heat of crystallization, would be obviated, and in addition, the processing efficiency of such recycled PET materials, and the resulting fabrication of high-performance strapping from such PET materials, can be effectively enhanced.
Accordingly, it is an object of the present invention to provide a new and improved inline solid state polymerization (SSP) process for processing PET flakes for subsequent processing of the same into high-performance plastic strapping.
Another object of the present invention is to provide a new and improved solid state polymerization (SSP) process, for processing PET flakes into high-performance plastic strapping, which obviates any operational or flow-through problems of the batch or charge of the PET materials attendant the processing of the PET materials within the solid state polymerization vessel.
Still another object of the present invention is to provide a new and improved solid state polymerization (SSP) process, for processing PET flakes into high-performance plastic strapping, which enhances the operational efficiency of the process by only using the more desirable crystalline flake components derived from the wall sections of the recycled PET materials whereby the solid state polymerization vessel facilities are optimally utilized so as to produce solid state polymerized materials which exhibit desirably high IV values, and the temperature level of the process is properly controlled so as to eliminate agglomeration and clumping of the materials which would otherwise cause flow-through problems of the batch or charge materials as well as clogging and jamming of the equipment components which would therefore result in production downtime of the equipment and increased maintenance costs.
The foregoing and other objectives are achieved in accordance with the teachings of the present invention through the provision of a process of directly converting post-consumer PET flake materials to materials having relatively high average intrinsic viscosity (IV) values whereby such resulting materials are useful in connection with the fabrication of particularly desirable products, such as, for example, high-performance strapping. High performance strapping exhibits increased weld strength. Weld strength is critically important in view of the fact that weld strength is often the weak link in strapping products. A weld strength value which is equal to 50% of the tensile strength of the strapping is considered normal for conventional or prior art high-performance strapping. As a result of the processing techniques of the present invention, however, the tensile strength, and accordingly the weld strength, of the strapping produced has been able to be increased approximately 30% with respect to the conventional or prior art high-performance strapping.
In accordance with the process of the present invention, the inventive process begins by obtaining post-consumer and non-post-consumer material containing PET. These materials may be obtained, for example, from strapping or material recovery facilities, and the materials have a relatively wide range of initial IV values, such as, for example, from 0.70 dl/g to 0.81 dl/g. The PET materials usually contain a variety of impurities, such as, for example, PVC, aluminum, polyethylene, polypropylene, and paper.
The PVC and aluminum materials are initially removed from the PET materials, and the PET materials are chopped into a heterogeneous mixture of flakes and chunks. As noted hereinabove, the material chunks are undesirable from the points of view of not being especially useful in enhancing the IV values of the PET materials, as well as adversely affecting the temperature level of the solid state polymerization process. In accordance with the specific teachings of the present invention, it is therefore desirable to remove such material chunks from the batch or charge of PET materials and such a process step is achieved by using a suitable destoner or sorter which effectively removes or sorts all or a large percentage of the chunk or neck portions of the PET materials from the flake or wall portions of the PET materials as a result of the different thickness and density properties of the chunk or neck portions of the PET materials as compared to similar properties characteristic of the flake or wall portions of the PET materials. As a result, essentially only desirable flakes or wall portions of the PET materials are further utilized within the solid state polymerization process so as to permit an enhanced volume of favorable or desirable PET materials to be processed, enhanced IV values of the processed PET materials is readily achieved, and the processing equipment is readily permitted to operate with a reduced amount of production downtime or production run interruptions as a result of the elimination of any agglomeration or clumping of the PET materials due to the desirably proper control of the operative processing temperatures.
After separation of the undesirable chunk or neck portions of the PET materials from the batch or charge of PET materials which now contains or comprises essentially only flake or wall portions of the PET materials, the PET materials are preheated within a fluid bed type dryer or preheater so as to undergo a preheating stage at a temperature level of approximately 315xc2x0 F. and a time period of approximately 20-25 minutes. As a result of such preheating process step, the PET materials are dried in view of molecular water having been removed therefrom. Subsequently, the PET flakes are now ready to enter the first stage of solid state polymerization, and accordingly, the PET flakes are placed into a hopper and heated in the absence of oxygen and in the presence of nitrogen until they reach a temperature level of between 390xc2x0 F. and 430xc2x0 F.
After undergoing the first stage of solid state polymerization for approximately one hour or more, the flakes are ready to enter the second stage of solid state polymerization, and accordingly, the heated flake mixture is removed from the hopper and placed within a bin in the absence of oxygen and in the presence of nitrogen. The flakes are heated to a temperature level of approximately 425xc2x0 F. and remain in the bin for a time period of approximately four hours.
Once the flakes have completed the first and second stages of solid state polymerization, the IV value of the resulting PET material has been increased to at least 0.90 dl/g, and to as high as 1.50 dl/g, withe the average IV value being approximately 0.95 dl/g. The PET flakes, having the enhanced IV values, can then be extruded through a suitable extruder so as to produce high-performance strapping. The strapping produced by means of the process of the present invention, that is, utilizing PET flakes which have been directly subjected to solid state polymerization, which have not necessarily been subjected to intermediate pelletization, and which have resulted in material having an enhanced average IV value of approximately 0.95 dl/g and a wide distribution of IV values within the range of 0.90 dl/g to 1.50 dl/g, is therefore able to comprise high-performance strapping which exhibits good tensile strength, weld strength, and joint strength characteristics. In addition, the solid state polymerization process to which the flakes of the present invention are subjected only requires a fractional amount of time that was previously required in connection with the prior art processing of the PET material pellets. As a result, in addition to the elimination of the substantially non-crystalline chunk segments or portions of the PET materials which enhances the flow-through processing of the materials and operational efficiency of the equipment without undergoing or experiencing operational interruptions or production downtime, the economical processing efficiency of PET materials, that is, the amount of time to process a particular batch or charge of PET material, is enhanced still further.